Use of mobile communications networks has tremendously increased over the last decade. Operators of mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of mobile communications networks. The base stations typically comprise radio systems for relaying radio signals, including software and hardware components. The radio signals are typically relayed into a cell of the mobile communications network. The operators of the mobile communications network wish to reduce the costs of the base stations. It is one option to implement the radio system as an antenna embedded radio system. With the antenna embedded radio system some of the hardware components of the radio system may be implemented on a chip. The antenna embedded radio system therefore reduces the costs of the base station. Implementing the radio system as the antenna embedded radio system reduces space needed to house the hardware components of the base station. Power consumption during normal operation of the radio system is substantially reduced when implementing the antenna embedded radio system comprising the chip.
It is of interest to provide a reliable quality of service to an individual user of the mobile communications network given the increase in the number of users. Several techniques have been suggested in order to deal with the increased number of users within the mobile communications network. One of the several techniques comprises beam forming capabilities in order to direct a beam relayed by the radio system in different directions to improve service coverage within the cells of the mobile communications network. The beam forming techniques rely on defined phase and amplitude relations between several of the antenna elements of the active antenna system. Calibration of transmit paths and receive paths is required to provide the defined phase and amplitude relationship between the beams. The calibration allows the estimation of a phase and amplitude deviation accumulated along the transmit path of the radio system. Likewise the calibration comprises estimating phase and amplitude deviations accumulated along the receive paths of the radio system. The calibration may further comprise a determination of transit times needed for a message signal to travel from the digital radio interface to the antenna element in order to be relayed. In a second step the phase and amplitude deviation accumulated along the transmit paths can be corrected. An appropriate phase and amplitude change may be applied to the individual transmit paths to yield the defined phase and amplitude relationship between the individual transmit paths of the radio system, in order to allow for beam forming techniques.
Applying the phase and amplitude changes to the transmit paths of the radio system strongly relies on transfer characteristics of the radio system being linear. Typically, an amplifier used within the transmit paths causes non-linearities within the transfer characteristics of the transmit paths. Analogue predistortion or digital predistortion are known methods for correcting the non-linearities of the transmit paths. It is of interest to provide the digital predistortion prior to the applying of the phase and amplitude changes. With significant non-linearities in the transfer characteristics of the transfer paths, the phase and amplitude changes will not yield the defined relative phase and amplitude relationship needed for the beam forming techniques.
The calibration of the phase and amplitude changes and the digital predistortion require a feedback path. The feedback path is in both cases used in order to evaluate any changes a radio signal undergoes when being relayed along the transmit paths. This holds for both a calibration signal as well as the payload signal being relayed by the radio system.
The prior art discloses two distinct feedback paths for calibrating the phase and amplitude changes and the digital predistortion. This requires time and it would be advantageous to calibrate the radio system faster and more efficiently. The two distinct feedback paths require additional complexity within the system and it is desirable to reduce this as far as possible.